I've not had Christmas lunch at Weatherspoons, but I've been there to eat and the food has been fine, and they have a good range of wines and beers. You've not said who these rellies are, but hopefully you could ask them to chip in with the cost.
We're going out to lunch with MiL and probably bro-in-law and his girlfriend on Xmas Day. It's just so much less hassle and allows us more time to relax and enjoy ourselves rather than faffing around with cooking. I'm doing mince pies, choccy yule log and various cold meats, pates and salads to have at home, and that's it.